The invention relates to a process for preparing gas separation membranes and modules. Gas separation membranes are films which exhibit a higher permeability toward some gases than toward others. Industries increasingly employ such membranes for such uses as: oxygen enrichment of air, purification of nitrogen, recovery of hydrogen from hydrogen reaction systems, removal of acid gases from natural gas or biogas, and separation of acid gases (Co.sub.2, H.sub.2 S).
Such membranes have no moving parts, negligible maintenance, low energy requirements, operational flexibility, and rapid start up and shut down. They may be formed as flat sheets, hollow fibers or tubes. A reinforcing fabric of woven or nonwoven polyester, or nylon increases membrane strength, improves operational properties and provides longer life.
The rate at which gas permeates a membrane can be expressed as p/1, where p is the gas permeability, and 1 is the effective membrane thickness. Cellulose acetate exhibits particular commercial importance because its effective thickness is about 0.1 micrometers. This in turn, results in a relatively high gas permeation rate.
Cloth reinforced membranes have been prepared by casting a cellulose acetate solution on the cloth and then gelling, leaching, annealing, solvent exchanging and drying the cloth/membrane combination. Cannon U.S. Pat. No. 3,497,072 discloses a phase inversion technique for casting a cellulose acetate membrane. If a wet membrane is to be dried directly, the high surface tension of water will destroy the microporous structure. Drying methods which can prevent this destruction include (1) freeze drying; (2) adding an agent to reduce surface tension; and (3) exchanging the water solvent for another solvent. McDonald et al U.S. Pat. No. 3,842,515 describes a solvent exchanging process which exchanges water with water missible alcohol, then exchanges the alcohol with a nonpolar organic solvent, and finally evaporates the nonpolar organic solvent to leave a dried membrane. Manos U.S. Pat. No. 4,080,743 describes a solvent exchanging process which first exchanges water with water missible organic solvents, and then evaporates the mixture to leave a dried membrane.
During the step of drying, different shrinkage ratios between the cellulose acetate membrane and the reinforcing cloth causes curling, wrinkling and cracking. Moreover, brittle membranes sometimes crack during winding into spiral wound modules. Schell U.S. Pat. No. 4,134,742 discusses these problems and attempts to reduce curling by utilizing a reinforcing cloth having a shrinkage ratio close to that of the membrane. This approach does not solve problems caused by uneven shrinking between the membrane in the reinforcing cloth, nor does it reduce cracking problems when winding a dried membrane into a spiral wound module.